This application relates to optical monitoring in optical devices and systems.
Optical properties or parameters of light in an optical device or system may be measured for various purposes. As an example, such an optical measurement may be used to determine the performance or an operating condition of the device or system. An optical property or parameter of light under measurement may include the optical polarization, the signal to noise ratio, the differential group delay between two orthogonal polarization states, and others. 
The optical polarization is an important parameter of an optical signal in various optical systems. For example, in fiber optic communication systems, polarization-dependent effects in fibers and other devices, such as polarization-dependent loss (PDL) and polarization-mode dispersion (PMD), can have significant impacts on performance and normal operations of optical devices or systems. Hence, it may be desirable to measure and monitor the degree of polarization (DOP) of an optical signal in these and others systems.
Similarly, the signal-to-noise ratio (SNR) and the differential group delay (DGD) of an optical signal are also important parameters and hence monitoring of these parameters may be desirable under certain circumstances. 